This invention relates to motor speed control systems, and more particulary to a speed control system applied to a motor of small size for minimizing speed variation due to load variation occurring during rotation of the motor at low speeds.
A speed control system for a series commutator motor is known in which a semiconductor element provided with a control electrode is disposed in the path of motor current supplied from an A.C. power source to the motor. In this known speed control system, a rectifier element is connected in parallel with the field winding of the commutator motor, and discharge current provided by the discharge of electrical energy accumulating in the field winding is used to excite the magnetic poles for increasing the electromotive force induced in the armature, thereby increasing the effect of feedback of the speed factor to the semiconductor element and improving the stability of speed control. A typical prior art speed control system of this kind is disclosed in U.S. application Ser. No. 261,099 entitled "Speed Control System for A.C. Series Commutator Motor," invented by Katsuji Soeda, filed June 8, 1972, issued Apr. 9, 1974 as U.S. Pat. No. 3,803,468, and assigned to the assignee common to the applicant of the present application.
Although the disclosed speed control system has been effective in improving the stability of speed control by increasing the electromotive force induced in the armature to increase the amount of feedback of the speed factor in the manner above described, occurrence of excessive load variation at low motor speeds has tended to give rise to considerable speed variation which is no more negligible. This phenomenon has been especially marked when a cam adapted to be incorporated in a motor-driven sewing machine is used to make pattern sewing on a fabric workpiece.